There is so much content in fandom regarding how Spite used to be Determination but I have a hot take here that I have been burning to share, here it is:
Spite is referred to/addressed multiple times as a spirit of determination, and crucially he does not correct this in the game. When Rowan addresses him as "Spirit of Determination" he's PUMPED, when Isabela introduces him in the Hall of Valor it's as "legendary Spirit of Determination, Spite!"
What I'm saying here is that Spite has always been Spite, and 'spite' is a type of determination. A subcategory, if you will. He is still a spirit of determination, just specifically determination as it manifests in the form of spite.
Taking this further, "demon" is a term that people in Thedas came up with to classify spirits they viewed negatively or as threats. So functionally, "demons" are just spirits that aren't benevolent or benign towards mortals, for whatever reason, whether they started out that way or were "corrupted" by circumstance/outside influence.
None of this is to say that Spite hasn't been traumatized or changed by his experiences, I think it's fairly obvious that he has, and not just in the Ossuary - I imagine living in an assassin's brain is going to have some impact on a being that is "susceptible to influence" (as Emmrich puts it) and could definitely explain Spite's being more inclined to solve problems with violence. But in spite of all this (see what I did there) from the very beginning Spite is remarkably cooperative for a so-called "demon." He makes an effort to understand and learn more about the world, he only ever takes control or lashes out in moments of intense frustration, he makes deals with Lucanis and adheres to them even when it would probably be easier to forcibly take him over, he actively bonds with other members of the team (Rook, Emmrich, Taash, Manfred) who bother to interact with him, he actively helps and shows concern for Lucanis and Rook even in situations that don't have anything to do with his own supposed goals. Spite didn't choose to be stuck in Lucanis, but he's determined to make things work (see what I did there again.)
Lucanis thinks he's making deals to appease Spite, Spite is actually making deals to appease Lucanis. And he's doing it because he feels whatever a spirit's equivalent of attachment is for him! The same thing that drew Manfred (Curiosity) to Emmrich, that drew the Spirit of Faith to Wynne all the way back in Origins, that draws the Wisps to Neve, etc. Lucanis is himself a human embodiment of determination, in at least the first half of the game that man is persevering on spite alone, and Spite loves that even if it means Lucanis is difficult as hell to reason with. Spite is frustrated that Lucanis keeps spiting him, but he also has to respect it. Game recognizes game.